At present, various cellular or mobile radio systems are in use or under development. When a mobile station (MS) is located in a cell, it communicates with the fixed network through a fixed radio station or base station located in the cell. The mobile stations belonging to the cellular system are allowed to roam freely within the area of the cellular system from one cell to another. The cellular radio network usually has information about the location of the MS with the accuracy of a so-called location area, which includes a suitable number of predetermined cells and their base stations. The location area data transmitted by the base station indicates to the MS the location area of the base station. When the MS changes a cell within the same location area, no updating of the location for the cellular radio network is necessary. Instead, when the MS detects on the basis of the location area data that the location area changes with the new base station, the MS initiates location updating by transmitting a location updating request to the cellular radio network. Due to the location updating request, the cellular radio network updates the new location area of the MS to a subscriber database.
As the location of the MS is known only with the accuracy of one location area, the MS has to be paged in all cells within the location area in question for the setup of an incoming call. This causes a considerable signalling load in the radio network between the mobile exchange and the base stations, as well as over the radio path. On the other hand, if the size of the location area is reduced to avoid the above drawback, the MS changes the location area more often; and so the location updating frequency of the subscribers and associated signalling increase. At present there is a tendency to enlarge location areas to reduce location updating.
When location areas are large, it may be advantageous to divide them into smaller paging areas. Since the location of the subscriber is known only with the accuracy of one location area, the first paging may even here be directed to a wrong location area; in the worst case the subscriber may not be found until the paging has proceeded to the last paging area. To reduce the signalling load, measures are needed even here to focus the paging of a subscriber on a small area and yet to maximize the probability of successful paging.